When to check Thyroid-Stimulating Hormone (TSH) levels after starting Synthroid (levothyroxine)?

TSH Monitoring After Starting Levothyroxine (Synthroid)

After initiating levothyroxine therapy, TSH should be checked at 4-6 weeks, with continued monitoring every 4-6 weeks until stable, then every 3-6 months initially, followed by annual monitoring once the patient is stable. 1

Initial Monitoring Timeline

• First check: 4-6 weeks after starting levothyroxine 1, 2
• Follow-up checks: Every 4-6 weeks until TSH stabilizes within target range 1
• Once stable: Monitor every 3-6 months initially, then annually 1

Dosage Adjustment Protocol

1. Initial assessment (4-6 weeks after starting):

   • Check TSH and free T4 levels
   • If TSH remains elevated: Increase dose by 12.5-25 mcg 1
   • If TSH is suppressed: Decrease dose by 12.5-25 mcg

2. Titration phase:

   • Continue adjusting dose in 12.5-25 mcg increments until TSH normalizes 1
   • Allow 4-6 weeks between dose changes due to levothyroxine's long half-life 1
   • Target TSH within normal range (0.45-4.5 mIU/L) for primary hypothyroidism 1

Special Populations Requiring Different Monitoring

Elderly Patients

• Start with lower dose (25-50 mcg/day)
• More frequent monitoring due to higher risk of cardiac complications 1
• May need more gradual titration

Pregnant Women

• Check TSH as soon as pregnancy is confirmed
• Monitor TSH at minimum once each trimester 1, 2
• Maintain TSH in trimester-specific reference ranges
• Increase dose as needed (typically by 25-50%)
• Return to pre-pregnancy dose immediately after delivery 2
• Recheck TSH 4-8 weeks postpartum 2

Common Pitfalls to Avoid

1. Premature dose adjustments:

   • Due to levothyroxine's long half-life, wait at least 4-6 weeks before adjusting dose 1
   • Day-to-day TSH variations of up to 50% can occur, so don't overreact to a single value 1

2. Medication interference:

   • Ensure patient takes levothyroxine on empty stomach, 30-60 minutes before breakfast 1
   • Check for medications that interfere with absorption (iron, calcium, antacids) 1
   • Adjust timing of interfering medications if needed

3. Overtreatment risks:

   • Excessive levothyroxine can lead to subclinical hyperthyroidism
   • Increases risk of atrial fibrillation and decreased bone mineral density 1
   • Symptoms include tachycardia, tremor, and sweating 3

4. Undertreatment risks:

   • Persistent hypothyroid symptoms
   • In children: adverse effects on cognitive development and growth 2

Interpretation of Results

• Persistent elevated TSH despite adequate dosing:

  • Consider absorption issues, medication interactions, or compliance problems 1
  • Assess timing of medication in relation to meals
  • Consider checking for celiac disease or other malabsorptive conditions

• Fluctuating TSH levels:

  • Ensure consistent timing of levothyroxine administration
  • Consider switching to liquid formulation if absorption issues suspected 4
  • Maintain consistent brand/formulation of levothyroxine when possible 5, 6

Remember that TSH is the most sensitive indicator of thyroid function, and the goal is to maintain it within the normal reference range while resolving clinical symptoms of hypothyroidism.